1. Field of the Invention
The present invention relates generally to oil field drilling operations and in particular to a method and apparatus that provides a user with enhanced control choke capabilities for directly controlling the pressure associated with and/or position of a choke element associated with a drilling choke during down hole operations.
2. Summary of the Related Art
There are many applications in which there is a need to control the backpressure of a fluid flowing in a system. For example, in the drilling of oil wells it is customary to suspend a drill pipe in the well bore with a bit on the lower end thereof and, as the bit is rotated, to circulate a drilling fluid, such as a drilling mud, down through the interior of the drill string, out through the bit, and up the annulus of the well bore to the surface. This fluid circulation is maintained for the purpose of removing cuttings from the well bore, for cooling the bit, and for maintaining hydrostatic pressure in the well bore to control formation gases and prevent blowouts, and the like. In those cases where the weight of the drilling mud is not sufficient to contain the bottom hole pressure in the well, it becomes necessary to apply additional backpressure on the drilling mud at the surface to compensate for the lack of hydrostatic head and thereby keep the well under control. Thus, in some instances, a backpressure control device is mounted in the return flow line for the drilling fluid.
Backpressure control devices are also necessary for controlling “kicks” in the system caused by the intrusion of salt water or formation gases into the drilling fluid, which may lead to a blowout condition. In these situations, sufficient additional backpressure must be imposed on the drilling fluid such that the formation fluid is contained and the well controlled until heavier fluid or mud can be circulated down the drill string and up the annulus to kill the well. It is also desirable to avoid the creation of excessive back pressures which could cause drill string to stick, or cause damage to the formation, the well casing, or the well head equipment.
However, maintenance of an optimum backpressure on the drilling fluid is complicated by variations in certain characteristics of the drilling fluid as it passes through the backpressure control device. For example, the density of the fluid can be altered by the introduction of debris or formation gases, and/or the temperature and volume of the fluid entering the control device can change. Therefore, the desired backpressure will not be achieved until appropriate changes have been made in the throttling of the drilling fluid in response to these changed conditions. Conventional devices generally require manual control of and adjustments to a choking device orifice to maintain the desired backpressure. However, manual control of the throttling device or choke involves a lag time and generally is inexact.
U.S. Pat. No. 4,355,784 (the '784 patent) discloses an apparatus and method for controlling backpressure of drilling fluid in the above environment, which addresses the problems set forth above. According to this arrangement, a balanced choke device moves in a housing to control the flow and the backpressure of the drilling fluid. One end of the choke device is exposed to the pressure of the drilling fluid and its other end is exposed to the pressure of a control fluid.
Conventional choke control systems are difficult to utilize accurately or efficiently and require a great deal of experience to operate properly. The typical conventional choke control mechanism comprises a needle valve to control the rate of hydraulic fluid flow and a direction lever for controlling the direction of an open/close valve in a choke device. For example, to make an adjustment to slowly increase the backpressure associated with a conventional choke, an operator shuts down the needle valve supplying hydraulic fluid to a hydraulically actuated choke to reduce supply of hydraulic fluid to a minimum so that the choke element moves slowly in the direction selected by the open/close valve. The operator relies on his experience in interpreting the familiar sounds and physical feedback associated with manipulating the choke controls. The operator relies on physical feed back during choke manipulation, that is, the resistance and vibration of the joystick and the sound of the air-over-hydraulic pump kicking in indicates to the operator that the choke control is engaged and operating. The operator looks at the backpressure and determines if the new desired back pressure was achieved. If the operator has overshot or undershot his pressure target, he similarly makes another adjustment using the open/close valve and the needle valve to adjust the choke until the desired backpressure is achieved. Proper adjustment of the choke element to achieve desired back pressure level is an iterative procedure typically requiring multiple attempts to achieve the desired result. This is a time consuming, inefficient and relatively inaccurate procedure for adjusting a choke. Thus, there is a need for method and apparatus for efficiently and accurately controlling a choke. There is also a need for a method and apparatus for directly controlling the position and back pressure associated with a choke device while maintaining the physical experience of traditional choke control methods to ensure proper operation by skilled operators experienced with conventional choke control methods. This method preferably is clear and straight-forward even to new (inexperienced) choke operators.